#ignimbrite

[drhoz]

The Great ACT-NSW-NZ Trip, 2023-2024 - Lake Taupō

I didn't just go to Aotearoa for the animals and plants - I wanted to see the geology as well. Australia hasn't done much apart from erode for the last 99 million years, but New Zealand is on the tectonic boundary between the Australian and Pacific plates, so the geology is much more active. In fact, in some areas it's positively lively.

This is Lake Taupō, a very large lake in the middle of the North Island, and the most active supervolcano in the world. The last big eruption, about 230 CE, blew out over 100 cubic kilometers of rock, much of it over seven or eight minutes. The previous one, 25750 years ago, blasted out over 10 times that. Large parts of the island were buried in hundreds of meters of red-hot volcanic ash, travelling at just under the speed of sound, that settled still hot enough to fuse into ignimbrite rock. The very term ignimbrite was coined in New Zealand to describe this kind of pyroclastic deposit. Microscopic diatoms from the lake sediments have been found mixed with the ash that landed in the Chatham Islands over 800km away. It's also made a nice dating layer in the Antarctic ice cap.

And if all this ash and the volcanic domes and geothermal fields around the lake weren't enough, the fact that the lake has black beaches, with a strandline of fist-sized pumice, and steep cliffs around the periphery, should be a big clue that this is not a healthy place to be. At least over geological periods. In theory there should be a little warning the next time the supervolcano decides to blow out a crater the size of Singapore.

@purrdence and I stayed at two places around the caldera - Taupo township, and a much smaller place on the southern end of the lake with a name that translates as 'Sandfly and all his mates'. Both venues had geothermal hot tubs attached. We also visited the Craters of the Moon geothermal field, and the Huka Falls cascade.

Craters of the Moon was certainly pungent - the numerous fumeroles and mudpots percolate a lot of sulphur, arsenic and other elements. One of the now-collapsed fumeroles, the Devil's Trumpet, used to be nocturnal pyrotechnic display, after guides threw kerosene-soaked sackng in and got a 50-foot column of fire in responce.

Some of the species I've seen before - most from Purrdence's visit to the area a year earlier, but also some from Australia. And at least two from my carport here in Perth.

[earthstory]

Fairy Chimneys These rock formations are found in the Cappadocia region of central Turkey. These towers are found within Göreme National Park just outside the city of Göreme.

All of the rocks you’re looking at are igneous. The lower layer is a thick ignimbrite layer, a layer of ash and pumice deposited during large, explosive volcanic eruptions. Those ash layers were then capped by a hardened, basaltic lava flow. Cracks in that lava flow allowed water to penetrate into the rocks and erode the lava flow. Once it got into the softer ash beneath the lava flow, the water was able to erode the looser ash and widen the spaces in-between. This process continued until all that was left was the occasional pillar of ash capped by basalt. An exchange between rhyolitic ashes and basaltic lava flows is common in igneous terrains. Basaltic lava is sourced from melting of the mantle but it is often too dense to reach the surface. Instead, it gets trapped in the crust and interacts with those rocks, producing more silica-rich rhyolitic magmas. On occasion basalt will still make it to the surface, producing areas like this one that alternate between explosive eruptions and basaltic lava flows. The area was declared a UNESCO World Heritage Site in 1985. -JBB Image credit: https://en.wikipedia.org/wiki/Göreme#/media/File:GoremeFairyChimneysTriple.jpg References: http://whc.unesco.org/en/list/357 http://bit.ly/1J7RfqQ http://www.cappadociaonline.com/nature.html http://www.fairychimney.com/english/guesthouse/location.htm

[goldpilot22]

gamer alignment chart ft. volcanic rocks [and rani]. i think this might be the first time i've posted some of them?

[relatetorocks]

“Fig-5. Photographs of intracaldera ignimbrite[s]”

[rocksncrystals-blog]

457 Green Tuff from Ischia, Italy 

weight 75g

[drhoz]

The Great ACT-NSW-NZ Trip, 2023-2024 - North Island Volcanic Plateau

Strictly speaking the Taupō caldera complex is just one part of the volcanic plateau covering much of central North Island, which also includes ash deserts, crater lakes, lava plateaus, the Ōkataina caldera complex, and the Tongariro Volcanic Centre. It's very active - Mount Ngauruhoe for example, is a beautifully cone-shaped stratovolcano 2,291 m tall, and less than 2500 years old. In fact the plateau is currently the most frequently active and productive area of silicic volcanism on Earth.

You might also recognise it as Mt Doom from the LOTR films.

In 1974 a champion skier (and presumed lunatic) by the name of Jean-Claude Killy skied down the side of Ngauruhoe you can see here, as part of a promotional campaign for Moët & Chandon. He hit well over 100kph. And he was doing this while the volcano was erupting.

Mount Ruapehu, south of Ngauruhoe, is also active, but is also the site of the North Island's only glaciers and major ski resorts. The eruptions have certainly been deadly, but most of the fatalities occured on Christmas Eve 1953 when the tephra dam constraining the crater lake collapsed and sent a landslide of mud, rock and water down the Whangaehu River minutes before an express train came through.

In 2007 two climbers nearly drowned on top of the mountain, when they took shelter in an alpine hut 600m from the crater, and an underwater eruption in the crater lake drove a surge 2km from the crater. Luckily for them the floor gave way and the water drained into the seismometer vault beneath the hut. One of the mountaineers still had a leg crushed by boulders in the debris.

East of the volcanoes is Te Onetapu, also known as the Rangipo Desert. It gets a decent amount of rain and occasionally heavy snow, but doesn't hold onto any of it because the entire area is thickly buried in unconsolidated ash and other volcanic debris and it immeadiately drains away.

It also doesn't have any trees to speak of, because the supervolcano eruptions at Lake Taupo 26,000 years ago carbonised every living thing in the area, including the seeds, and the ecosystem is still recovering. Forests on the western side of Ruapehu were partly shielded by the height of the volcano, and survived.

While @purrdence spent a few hours at the military museum south of the desert, I drove back up the road and went looking for insects. Some were abundant, like the Tiger Beetles, but they amazingly frustrating to photograph, taking off and being whisked away by the wind the moment I came near. Or being chased off, sometimes in flight, by the aforementioned ravenous beetles. At least the plants stayed in one spot, and the bumblebee was enthralled by the blue paintjob of the car.

[earthstory]

Fiamme!

This rock is a “welded tuff”. Volcanoes explode when gases trapped in bubbles inside of molten rock build pressure and explode, tearing the molten rock apart. That explosion forms fine-grained particles called ash and larger chunks that still hold the shape of the bubbles called pumice.

Those particles fly through the air but they’re still molten rock, they’re still hot. If they land in a big enough pile, the heat in the ash grains can cause them to reshape. The pile of hot ash will literally turn itself back into a rock that we call a welded tuff.

Larger chunks of pumice also get incorporated into these tuffs as they weld. A piece of pumice is mostly open space with a tiny bit of rock in-between. Heat that pumice up and it loses all strength – the weight of the rock on top of it will squeeze a piece of pumice down into the flat layers you see here. Those flattened pumice are called fiamme. Here, the fiamme are a little weaker than the surrounding tuff, so they’re also eroding out and leaving open spaces.

-JBB

Image credit: http://bit.ly/1AVB9iR

Read more: http://bit.ly/1BNNbhO

[goldpilot22]

who gave the baby a welding torch